DNA repair systems identified in mitotic calls of the yeast Saccharomyces cerevisiae are being examined for their protection of cells undergoing meiosis and the role of the corresponding genes in normal meiosis. The RAD50, RAD52 and RAD57 genes are essential in the repair of DNA double-strand breaks in mitotic cells. We have shown that they are also required for meiosis. Mutations abolish normal meiotic recombination; RAD50 acts early in meiosis. Rare single-strand interruptions (SSIs) are observed in rad52 and rad57 strains which appear to be related to recombination and these have been characterized. Based on genetic and biochemical changes, the order of gene function appears to be RAD50, RAD52, and RAD57. Given the important role RAD52 plays in repair and recombination, we have initiated studies to characterize its function in normal DNA metabolism and following treatment with DNA damaging agents. This is being done by "domain mapping" the functional regions of the RAD52 gene. Included among the processes affected by RAD52 are growth, recombination, mutagenesis, control of the essential yNUCR gene, control of strand exchange protein levels in meiosis, meiotic cellular viability, post replication repair, and DNA strand breaks in meiosis. Mutations are being created in vitro and the altered gene is being transplaced into the genome. The consequences to the above gene functions are being examined. In addition, the role of the gene and mutants in relation to gene dosage is being examined using a system developed in this laboratory for isolating cell lines with different numbers of the gene.